1. Field of the Invention
The present invention relates to a process for manufacturing monochloroacetaldehyde trimer together with chloral.
2. Description of the Background Art
Monochloroacetaldehyde (hereinafter abbreviated as MCA) is a compound useful as a raw material for the synthesis of organic compounds such as drugs and agricultural chemicals. Because this compound is extremely unstable and polymerizes very easily, it has been difficult to store it as is for a long period of time. Because of this, MCA is stored as an aqueous solution of about 40%. It has beer found that MCA can be stored stably, if trimerized into a trimer and stored as the trimer, and the trimer can be regenerated into high purity MCA. A process for the manufacture of MCA trimer has been proposed by Japanese Patent Laid-open (kokai) No. 223575/1990. This process comprises chlorinating acetaldehyde or para-aldehyde to a degree at which MCA can be obtained at a highest concentration, removing low-boiling point components by bubbling an inert gas such as nitrogen gas, dissolving the chlorinated liquid containing MCA as a major component, and trimerizing the MCA in the presence of high concentrated sulfuric acid.
In this process the chlorination of the raw material acetaldehyde or para-aldehyde is carried out under the conditions which can bring about the highest MCA concentration, while suppressing production of dichloroacetaldehyde (hereinafter abbreviated as DCA). However, the amount of acetaldehyde unreacted is increased when reacted under such conditions. Separation of this unreacted acetaldehyde by distillation accompanies production of high-boiling point components due to condensation reactions among aldehyde molecules, making it difficult to reclaim acetaldehyde which has not been chlorinated. The yield of MCA by distillation is also reduced. Furthermore, this process has another problem in that it requires washing the solution containing MCA trimer with water in order to remove concentrated sulfuric acid which is used as a catalyst for trimerizing MCA after removal of non-chlorinated acetaldehyde. When the solution containing MCA trimer is washed with water, non-trimerized aldehydes, which are abundantly soluble in water, are transferred to the water phase. It is almost impossible to reclaim the aldehydes transferred to the water phase. The utilization of raw material acetaldehyde and para-aldehyde in this process is thus extremely low.
In view of this situation, the present inventors have undertaken further studies in order to more effectively manufacture MCA trimer, and found that this objective can be achieved by a process comprising chlorinating acetaldehyde or para-aldehyde to a non-chlorinated acetaldehyde concentration of 5% or less, distilling the produced MCA in the presence of chloral, trimerizing the MCA, and converting the non-trimerized acetaldehydes into chloral.